Many methods of separating particulate materials from bulk materials and slurries employ a vibrating sieve, over which material is moved so that smaller particles may pass through the openings while the larger particles pass therealong. The sieve is activated by one or several vibrators which generate vibration of the sieve surface at predetermined frequency and amplitude. These methods are generally ineffective, however, when applied to certain materials, such as wet materials, fine powders with a significant tendency to agglomerate, highly cohesive powders, and generally, so-called difficult to sieve materials; and the sieves used for sieving of such materials are prone to clogging.
There are also known separation methods based on vibratory excitation of the sieve surface by two or more vibrators applying simultaneous superposed vibrations with different forced frequencies and amplitudes to a sieve, and consequently, to material providing for screening thereby. These methods provide an increase in screening efficiency and a reduction in clogging of the sieve meshes. Such methods employ apparatus having two or more vibrators for driving a screen, wherein one or more vibrators provide low frequency vibratory excitation of the screen, with a relatively large amplitude, while one or more other vibrators provide vibration of a smaller amplitude, and at a higher frequency.
By way of example, U.S. Pat. No. 5,232,099 discloses a screening apparatus and method, wherein there are provided low amplitude vibrations having a frequency in the range 1000-7000 vpm, and at an amplitude in the range 600-1350 rpm. High frequency vibrations, which are generated by several vibrators, are applied directly to a sieve, while low frequency vibrations are transmitted to the sieve via a housing. An advantage of this method is relatively low wear of the sieve and an increased capacity. Disadvantages include the necessity of provision of several additional vibrators; and the concentration of vibratory energy transmitted to sieve, and subsequently to the material, within narrow frequency zones. The absence of wide band spectrum excitation hinders desired disintegration of powder agglomerates.
There are also known vibration separation methods which employ double frequency excitation. These methods combine use of a low frequency vibration, typically in the range 5-60 Hz, with ultrasonic excitation, provided by means of an electromechanical transducer fed by an electronic generator, which provide high-frequency sine vibrations of a sieve. Such vibration is typically within the 20-50 kHz band, with a small amplitude. Ultrasonic vibration may be either continuous or intermittent. By way of example, the SONOSCREEN system of Telsonic AG, Bronschhofen, Switzerland, is an ultrasonic sieving system for fine powders, having ring-shaped resonators to provide an even micro-oscillation. In more detail, the sieve separator has a sieve assembly which includes a sieve frame, and a sieve fabric which is tensioned and bonded to the frame. The ultrasonic transducer is rigidly fastened to the frame inside the screening area. The frame has a natural frequency which is close to the forced frequency of the ultrasound generator. The sieve assembly is fixed inside the separator housing, thus providing combined vibration of the screen deck at both low and high frequencies. Disadvantages of this system include relatively low energy transfer to the sieve fabric, and insufficient de-agglomeration efficiency.
Another example of a screening system employing double frequency excitation, is provided by U.S. Pat. No. 5,542,548. In this patent, there is provided a screening system having a resiliently mounted frame, a low frequency vibratory drive coupled to the frame, a screen extending across and resiliently mounted to the frame, and a plurality of high frequency drives mounted rigidly to the periphery of the screen, operative to vibrate the screen at a frequency in the approximate range 10,000 to 50,000 Hz. The screen is adapted to be responsive to high frequency vibrations in a plate-like manner. Fine mesh screens may be supported by backing screens, coarse mesh screens or perforated plates, bonded or unbonded.
Various systems, generally similar to the above system, employ an ultrasonic transducer, this being directly fastened via a washer to the center of a round sieve. One such system is the Vibrasonic® 2000 Mesh Deblinding System, manufactured by Russell Finex Limited, of Russell House, Browells Lane, Feltham, Middlesex TW13 7EW, England.
A further type of system is exemplified by the Ultrasonic Circular Screen Separator, manufactured by Kason Corporation, 67-71 East Willow St. Millburn, N.J. 07041-1416, U.S.A. This separator combines low and high frequency excitation of a sieve, by means of a pair of ultrasonic transducers which act on the sieve surface, via a pair of metal rings. The transducers, which transform electric signals provided by a electronic generator, are fastened to a support on the screen frame. The transducer's downward force and position can be adjusted. This makes possible the provision of dual vibration excitation to the sieve fabric of standard screens. Furthermore, there is provided a low frequency vibration via a vibratable housing, by means of a coaxially aligned, unbalanced vibrator motor; and a high frequency vibration, directly applied to the sieve fabric from an ultrasonic transducer. The VORTI-SIV® division of MM Industries, Inc., of 36135 Salem Grange Road P.O. Box 720, Salem, Ohio 44460, manufactures an Ultrasonic De-blinding System, which superimposes an ultrasonic high frequency excitation on a low frequency excitation sieving system.
Among the main disadvantages of the above-described dual-frequency separation methods is the absence of wide band spectrum excitation, and a low transference of mechanical energy to the screen, causing low performance.
Yet further known is the unblocking of sieves by impact action of different shock means upon a sieve element. Disclosed in U.S. Pat. No. 5,301,815 is a screening device, which comprises a vibrating frame to which a vibrating housing is attached, supporting a vibrating screen. Below the screen are attached a number of fixed bars, as well as a number of movable bars, each held at the extremity of two arms attached by a flexible connection. The vibrating housing is driven by a first electric motor used during the normal screening process and a second electric motor which rotates at a lower speed than that of the other motor for unblocking the device by maintaining the movable bars in resonance so that they strike the lower surface of the vibrating screen.
U.S. Pat. No. 4,288,320 describes unclogging a sieve in a vibratory screen by use of a plurality of weighted springy arms. The arms are formed integrally with and so as to extend laterally, to either or both sides of a resilient mounting strip that is disposed in touching contact with and beneath, the lower surface of the screen. The vibrating action of the vibratory screen induces an oscillating movement in resiliently mounted arms so as to give rise to a rapping action of the weighted ends of the arms against the undersurface of the screen, thereby to loosen material plugging the screen openings, and so as to unclog them. A disadvantage of this arrangement is the intensive wear of the sieve and arms caused by the motion of these elements when abrasive materials are processed.